bhyve: nvme controller obey async event setting when reporting critical temperature

Async event report is controlled by async event configuration feature
setting. When reporting a critical temperature warning, check the async
event configuration.

Approved by: manu (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37355

(cherry picked from commit 10846c53c4569bdf6d6e2b88739a56a8273bce7f)